It has been previously described that a hydrocarbon feed such as methane can be converted into a product stream comprising more complex hydrocarbons by a process wherein the hydrocarbon feed is first reformed to synthesis gas (syngas) followed by a second step wherein said syngas is converted into a hydrocarbon product stream by Fischer-Tropsch synthesis (F-T synthesis).
In WO 03/016250, for instance, such a process is described wherein first a mixture of a hydrocarbon feedstock and steam is subjected to catalytic steam reforming to produce a primary reformed gas, which then is partially combusted with an oxygen-containing gas over a secondary reforming catalyst to produce syngas. Said syngas is used to synthesize hydrocarbons by F-T synthesis. The process according WO 03/016250 is characterized in that the tail gas obtained in the F-T synthesis step is recycled to the primary reformed gas.
In FR 2878858 a combined process for producing hydrocarbons is described that comprises a reforming step wherein a hydrocarbon feedstream is converted into syngas and a F-T synthesis step wherein the produced syngas is converted into a hydrocarbon product stream. The syngas in FR 2878858 is preferably produced by a technology that employs oxygen, such as catalytic autothermal reforming or (catalytic) partial oxidation.
In WO 2006/097440 a process to prepare a F-T synthesis product is described in which a syngas mixture is prepared by (a) pre-reforming a hydrocarbon feed to produce a mixture of methane, hydrogen and carbon dioxide; (b) heating of the mixture obtained in (a) to above 650° C.; and (c) performing partial oxidation of the heated mixture of (b) with a source of oxygen.
A drawback of conventional processes for producing hydrocarbons that combine a reforming step and an F-T synthesis step is the non-selective production of carbon dioxide and methane in the F-T synthesis step. This leads to a relatively low C2-C6 hydrocarbon selectivity and renders the process not economical.
The technical problem underlying the present invention is the provision of a process for converting methane (CH4) to a mixture of aliphatic and aromatic C2-C6 hydrocarbons that has an improved selectivity for aliphatic and aromatic C2-C6 hydrocarbons.